1. Technical Field of the Invention
The present invention relates generally to a dielectric coaxial resonator which may be employed in a wide variety of radio communication devices, and more particularly to an improved structure which produces a dielectric coaxial resonator of very small physical size with a high unloaded Q.
2. Background Art
In recent years, there is an increasing need for compact and lightweight equipment in the field of radio communication such as portable telephones. In filter devices commonly built in present day portable radio communication devices, coaxial resonators using dielectric materials assuring a high dielectric constant with low loss, are widely utilized. The reduction in size for such coaxial resonators is usually accomplished by using dielectric materials having a high dielectric constant or modifying the shape of a resonator body so as to change the characteristic impedance of a line in a stepwise fashion.
FIGS. 7(a) and 7(b) show a conventional dielectric coaxial resonator. FIG. 7(a) illustrates a vertical cross section of the coaxial resonator taken along the center line thereof. FIG. 7(b) is a side view.
The shown coaxial resonator generally includes a hollow dielectric substance 1 having formed therein a through hole 2, a ring portion 3, a central conductive film 4 continuing from the ring portion 3 to the through hole 2, and an outer conductive film 5 to produce a structure wherein one end is opened and the other is short-circuited.
The dielectric coaxial resonator thus constructed provides an increased inductance component of the central conductive film as well as increased capacitive components between the ring portion 3 and the through hole 2 and between the through hole 2 and the outer conductive film 5, thereby allowing the overall size to be reduced.
The above prior art resonator, however, has the drawback in that shortening the full length of the resonator requires the formation of a plurality of ring portions in the opening end portion or the increase in depth of the ring portion, thereby resulting in an increased area of the central conductive film exposed to the outside as well as complex machining processes. This causes electric field components around the opening end portion of the resonator to spread out of the outer conductive film 5, leading to the reduction in unloaded Q.
Additionally, the adjustment of the resonance frequency is conventionally accomplished by machining the outer conductive film. It is, however, difficult to adjust the resonance frequency while maintaining the axially symmetrical structure as is, leading to the uneven distribution of electromagnetic field, which will cause the unloaded Q to be reduced.